1. Field of the Invention
The present invention relates to a device for detecting and suppressing resonance in a passage conducting a high temperature gas, such as an exhaust passage of an automotive vehicle. More specifically, the present invention relates to an arrangement wherein exhaust system can be selectively tuned in a manner which suppresses resonance.
2. Description of the Prior Art
In internal combustion engines, having exhaust systems according to the prior art, it has been found that during the period before the engine is completely warmed up, there is a tendency for the engine to produce a loud, high pitched, metallic sound, when accelerated at certain engine speeds. This sound generally occurs in the 1 to 10 kHz range. Although the exact mechanism by which this sound is produced is still uncertain, its existence and the resulting resonance it causes in the exhaust system have been confirmed and measured.
The problem of the sympathetic resonance occurring in the exhaust system as a result of this sound, is particularly pronounced in high power engines having low back pressure exhaust systems.
It has also been found that the production of this high pitched sound is not limited to just the periods when the engine is being accelerated before it is completely warm, but also occurs under other transitional engine operating states. For example this sound also tends to be produced when the engine is being decelerated under certain conditions. One such condition is during engine braking.
In such engines it is of course possible to reduce the noise from the exhaust by increasing the amount of baffling in the mufflers. In such cases however, as the baffling is increased, the noise is reduced but the power output and fuel efficiency of the engine are reduced as well. For these reasons there is a clear demand felt for an exhaust system wherein the resonances of the engine and exhaust system can be suppressed with a minimum of energy consuming restrictions to the flow of exhaust gasses through the exhaust system.
In order to suppress the noise produced by the engine under these types of conditions, while incurring a minimum of restriction to the flow of exhaust gas, a system disclosed in JP-A-63-82022 was proposed. In this system, the points along the length of the exhaust system that defined the nodes of sound waves at resonant frequencies were determined, and the passage was provided with flow restrictions at the these points. These flow restrictions were intended to reduce the resonance of the exhaust passage at particular frequencies.
In the above system, the flow restrictions were comprised of butterfly valves. Operating these valves enable the selective restriction of the exhaust gas flow at the resonance nodes in a manner which altered the resonance characteristics of the exhaust passage.
Other examples of prior art methods for tuning the resonant frequencies of an exhaust system in order to suppress resonance, are given in JP-A-48-39453 and JP-A-62-57718. In these two documents systems are disclosed in which the exhaust system comprises a plurality of passages of mutually different lengths, and a selector arrangement for selecting which of the passages is open at a given time. By the selection of the exhaust passages, it was attempted to select the effective length of the exhaust system so as to alter the resonating frequency and thus suppress resonance in the exhaust system.
In the respective prior art systems, the engine speed and the engine load were monitored and the exhaust tuning arrangements were controlled on the basis of control values derived as a function of these parameters. For example, at a low engine speed and low engine load, a particular setting would be established arithmetically on the basis of the detected parameters, and what was hoped to be a non-resonant frequency, would be selected for the exhaust system. Conversely, at higher loads and engine speeds the exhaust system was tuned to another frequency.
A problem has been encountered in the types of resonance suppression systems outlined above. The resonating characteristics of the exhaust system have been found to vary with temperature. Because of this, it is impossible, on the basis of the sensed engine speed and engine load alone, to accurately determine the proper timings for operation of the flow restriction valves under all engine operating conditions. Further, in the above systems there was no way of detecting whether or not the frequency of noise which was intended to be suppressed, was actually the one occurring. Therefore, erroneous and/or ineffective operation of the valves tended to result. In some instances this not only failed to suppress the resonance, but actually tended to enhance it, and accordingly increased the level of the noise produced by the engine.
In order to overcome the above problem, it is conceivable to provide a sound pressure sensor in the exhaust passage, by which it would be possible to detect whether or not the resonance is occurring. Providing such a sensor however, induces the problems that the sensor must be highly accurate, and must be able to withstand high temperatures and pressures. For the purposes of experimentation, such a sensor is not difficult to obtain, however with regard to production designs wherein such a sensor is incorporated in an exhaust system, the cost for a sensor having adequate durability has been determined to be too high to be practical.